This invention relates to a liquid crystal display unit and, more particularly, to a liquid crystal display unit with a spacer and a process for fabricating thereof.
The liquid crystal display unit has a pair of transparent substrate structure spaced from each other, and liquid crystal is filled in the gap between the transparent substrate structures. An active matrix liquid crystal display unit is popular, and has the following structure. Pixel electrodes are arranged on one of the transparent substrate in matrix, and thin film transistors are connected between data lines and the pixel electrodes. The pixel electrodes are, by way of example, formed of indium tin oxide. A common electrode is provided in such a manner that electric field is created between each pixel electrode and the common electrode. A pair of polarizing plates is incorporated in the active matrix liquid crystal display unit. Black light passes one of the polarizing plates before entry into the liquid crystal, and images are produced through the other polarizing plate after passing the liquid crystal. The pixel electrode, the common electrode and a piece of liquid crystal form in combination a capacitive pixel. The piece of liquid crystal serves as a dielectric layer of the capacitor. The transparency of the piece of liquid crystal is varied depending upon the intensity of the electric field created in the capacitive pixel as follows.
The thin film transistors are sequentially changed to on-state, and the data lines are electrically connected through the thin film transistors to the associated pixel electrodes. Image-carrying signals are supplied to the data lines in synchronization with the selection of the pixel electrodes, and change the potential level on the pixel electrodes. Electric fields are created between the pixel electrodes and a common electrode, and pieces of liquid crystal over the pixel electrodes are selectively twisted so as to change the transparency of the pixels. Only predetermined polarized light passes the selected pixels. The matrix of pixels partially transfers the back light, and partially interrupts it. As a result, a picture is produced on the matrix of pixels. Thus, the picture is produced on the matrix of pixels through the electrooptic anisotropy.
The matrix of pixels occupies most of the major surface of the substrate structure. However, the peripheral area is not available for the image production. The area assigned to the matrix and the peripheral area are herein-below referred to as xe2x80x9cimage producing areaxe2x80x9d and xe2x80x9cframexe2x80x9d, respectively. The active matrix liquid crystal display unit includes a black matrix and color filters. The black matrix is provided for enhancement of the contrast. The pixels are surrounded by the black matrix, and the black matrix prevents the pixels from leakage light. The color filters are usually provided on the transparent substrate opposite to the transparent substrate for the pixel electrodes and the thin film transistors. A red filter, a green filter and a blue filter are respectively associated with the three pixel electrodes, and colored images are produced on the matrix of pixels with the assistance of the colored filters.
The active matrix liquid crystal display units are categorized into two groups. The first category is featured by the common electrode formed on the transparent substrate opposed to the other transparent substrate where the pixel electrodes and the thin film transistors are formed. An active matrix liquid crystal display unit of the first category is referred to as xe2x80x9ctwisted nematic type liquid crystal display unitxe2x80x9d. On the other hand, the other category is featured by the common electrode formed on the transparent substrate together with the pixel electrodes and the thin film transistors. An active matrix liquid crystal display unit of the second category is referred to as xe2x80x9cin-plane switching type liquid crystal display unitxe2x80x9d. The electrooptic anisotropy is used in both twisted nematic and in-plane switching type liquid crystal display units for producing images.
As described hereinbefore, the pixel electrode, the common electrode and the piece of liquid crystal form in combination the capacitive pixel, and the transparency is varied depending upon the intensity of the electric field created in the capacitive pixel. If the liquid crystal layer is changed in resistivity and/or thickness, the capacitance of the pixel is varied, and the picture on the matrix becomes less clear. On the other hand, if the liquid crystal layer is constant in resistivity and thickness over a long time period, the active matrix liquid crystal display unit continuously produces fine images on the matrix of pixels.
The thickness of liquid crystal layer is equal to the gap between the substrate structures. If the gap is constant, the liquid crystal layer does not change the thickness. On the other hand, if the gap is changed, the liquid crystal layer also changes the thickness. For this reason, a spacer is provided between the substrate structure for keeping the gap constant.
The spacer is, by way of example, implemented by micro-pearls scattered in the substrate structure at a predetermined density. Another spacer has a column shape. Photo-sensitive synthetic resin is spread over the substrate structure, and the photo-sensitive synthetic resin layer is selectively exposed to light. A latent image is produced in the photo-sensitive synthetic resin layer. When the latent image is developed, the column-shaped spacer is left at predetermined positions on the substrate structure. Yet another spacer is produced in the assembling stage of the liquid crystal display unit. Micro-pearls are mixed in sealing agent. The sealing agent is spread over the periphery of the substrate structure and predetermined area. The micro-pearls are spread together with the sealing agent. The other substrate structure is aligned with the substrate structure, and is assembled therewith. The micro-pearls keep the substrate structures spaced from each other for producing the gap.
However, the inner surfaces of the substrate structures are not flat due to the signal lines, electrodes and color filters. In other words, the substrate structures have rolled inner surfaces. If the substrate structures are assembled in such a manner as to oppose the rolled inner surfaces to each other, the gap is varied, and, accordingly, makes the liquid crystal layer varied in thickness. In order to make the inner surface flat, soft coating layers are spread over the rolled inner surfaces of the substrate structures. The signal lines, electrodes and color filters are covered with the soft coating layers, and the inner surfaces of the substrate structures become flat. This results in a constant gap.
FIGS. 1 and 2 shows a typical example of the in-plane switching type liquid crystal display unit. The prior art liquid crystal display unit is broken down into a pair of substrate structures and liquid crystal confined between the lower substrate structure and the upper substrate structure. Thin film transistors are incorporated in the lower substrate structure, and color filters are formed in the upper substrate structure.
The lower substrate structure is fabricated on the basis of a transparent substrate 100. Gate electrodes 3 and a common electrode 4 are formed on the transparent substrate 100, and are covered with an insulating layer 10. Data lines 5, a drain electrode 7, a source electrode 6 and a pixel electrode 2 are patterned on the insulating layer 4, and are converted with a passivation layer 8. The gate electrodes 2 extend in perpendicular to the data lines 6, and thin film transistors are assigned to regions where the gate electrodes 2 cross the data lines 6. Amorphous silicon layers 1 are formed in the regions on the insulating layer 10. The amorphous silicon layer 1 has a drain region, a source region and a channel region, and the drain electrode 7 and the source electrode 6 are held in contact with the drain region and the source region, respectively. The drain electrode 7 is merged with the associated data line 5, and is spaced from the source electrode 6. The source electrode 6 is merged with the pixel electrode 2. The pixel electrode 2 is offset from the common electrode 4, and is in parallel thereto. An orientation layer 11 is formed on the passivation layer 22, and a polarizing plate 18 is attached to the lower surface of the transparent substrate 100. The data lines 5, the drain electrodes 7, the source electrodes 6 and the pixel electrode are shown in black in FIG. 1 for easily discriminating them from other electrodes.
On the other hand, the upper substrate structure has a transparent substrate 200, and a black matrix 12 and colored layers 13 are formed on the lower surface of the transparent substrate 200. Apertures H are defined in the black matrix 12, and are covered with the color filters 13. The black matrix 12 and the colored layers 13 are covered with a flattering layer 15, and an orientation layer 11 is formed on the lower surface of the flattering layer 15. The upper surface of the transparent substrate 200 is covered with a conductive layer 16, and a polarizing plate 17 is attached to the upper surface of the conductive layer 16.
The upper substrate structure is spaced from the lower substrate structure in such a manner that the orientation layers 11 are opposed to each other, and the liquid crystal 20 fills the gap between the orientation layers 11. The orientation layers 18 was subjected to a rubbing at a certain angle with respect to the longitudinal direction of the pixel electrode 2, and the liquid crystal 20 is oriented in a direction indicated by arrow 19.
A spacer is incorporated in the prior art in-plane switching type liquid crystal display unit. FIG. 3 shows the spacer. The spacer is implemented by micro-rods 23 and micro-pearls 25. The micro-rods 23 are mixed in sealing agent 24, and, accordingly, is provided under the frame. The sealing agent 24 is formed of epoxy resin. On the other hand, the micro-pearls 25 are assigned the image producing area.
The micro-pearls 25 are formed of glass, and have a spherical shape. Prior to the assemblage, the micro-pearls 25 are scattered over the orientation layer 11 of one of the substrate structures. The substrate structures are assembled with each other. Then, the micro-pearls 25 are sandwiched between the orientation layers 11 in the image producing area, and keep the substrate structures spaced. The micro-rods 23 are mixed with the sealing agent 24, and is spread in the peripheral area of the substrate structure. When the substrate structures are assembled together, the micro-rods 23 keep the substrate structures spaced. Thus, the micro-pearls 25 cooperate with the micro-rods 23, and the gap is formed between the substrate structures. When the liquid crystal 20 is injected into the gap, the sealing agent 24 prohibits the liquid crystal 20 from flowing out.
FIG. 4 shows another spacer available for the prior art in-plane switching type liquid crystal display unit. In this instance, the spacer is implemented by the micro-rods 23 and spacer columns 26. The micro-rods 23 are mixed in the sealing agent 24, and keeps the peripheral areas of the substrate structures spaced as similar to those shown in FIG. 3. The spacer columns 26 are assigned the image producing area. The spacer columns 26 are formed under the black matrix 12, and are substantially aligned with the data lines 5. The spacer column 26 is formed of acrylic resin, which is used for the flattering layer 15. An acrylic resin layer is patterned into the spacer columns 26 by using photo-lithographic techniques. The flattering layer 15 and the spacer columns 26 are covered with the orientation layer 11. When the substrate structures are assembled together, the spacer columns 26 are provided between the substrate structures in the image producing area, and keep the substrate structures spaced.
A problem is encountered in the prior art liquid crystal display unit in that irregularity is liable to take place in a picture produced on the image producing area.
It is therefore an important object of the present invention to provide a liquid crystal display unit, which has a sealing layer effective against contaminant.
It is also an important object of the present invention to provide a process for fabricating the liquid crystal display unit.
The present inventor contemplated the problem inherent in the prior art liquid crystal display unit. The present inventor investigated the contamination, and found that the irregularity was due to contamination of the liquid crystal. The present inventor further investigated the contamination mechanism, and found out that the origin of the contamination.
The liquid crystal was injected into the gap between the substrate structures after the assemblage. The assembled substrate structures tended to be contaminated with excessive liquid crystal, and the manufacturer washed the liquid crystal way with acid solution. Grooves D were formed along the edges of the substrate structure (see FIG. 5), and the acid washing solution was liable to remain in the groove. The sealing agent was in the epoxy resin system. The epoxy resin was liable to be damaged in acid solution. In fact, cracks C took place after the cleaning. As a result, ionic solution, i.e., electrolyte penetrated through the cracks C into the liquid crystal. The ionic solution or contaminant reduced the resistivity of the liquid crystal, and the pixels were varied in capacitance. The potential differences between the pixel electrodes 2 and the common electrode 4 were lost within a short time, and the picture produced on the image producing area fluctuated. Even though the ionic solution could not reach the liquid crystal, the environment contaminated the liquid crystal with humidity in a long operating time.
If the sealing agent was made wider, the cracks C could not reach the liquid crystal layer. However, the wide sealing agent layer resulted in a wide frame, and the wide sealing agent layer was against the request for a narrow frame. Although sealing agent in acrylic resin system withstood the ionic solution, there were only several kinds of acrylic sealing agent presently obtainable in the market, and were not always appropriate to the selected liquid crystal.
To accomplish the object, the present invention proposes to form a spacer of anti-erosion material in a peripheral area. The anti-erosion material is less damaged in the washing solution rather than the sealing agent. The anti-erosion property may be effective against the erosion through ionic reaction. Epoxy resin is, by way of example, used as sealing agent. When residual liquid crystal is washed away by using acid washing solution, acrylic resin is resistive against the acid washing solution, and the spacer may be formed of acrylic resin.
In accordance with one aspect of the present invention, there is provided a liquid crystal display unit for producing visual images in an image producing area comprising a pair of substrate structures providing the visual image producing area and a frame area around the image producing area and including electric components, a sealing layer extending along the frame area for forming an inner space between the substrate structures of the pair and formed of a first kind of material, liquid crystal filling the inner space and varying a transparency depending upon electric signals on the electric components for producing the visual images, and a spacer for spacing the substrate structures from one another and including a spacer wall structure extending in the frame area for encircling the inner space and formed of a second kind of material less eroded rather than the first kind of material in a washing solution used for washing residual liquid crystal from an outer surface of the pair of substrate structures.
In accordance with another aspect of the present invention, there is provided a process for fabricating a liquid crystal display unit comprising the steps of a) preparing a pair of substrate structures having an image producing area and a frame area around the image producing area, at least one of the substrate structures of the pair including a spacer having a spacer wall structure extending along a looped sub-area in the frame area and formed of a kind of material less eroded rather than a sealing agent in a washing solution, b) forming a layer of the sealing agent along the looped sub-area, c) completing the liquid crystal display unit filled with liquid crystal in an inner space between the substrate structures, and d) removing residual liquid crystal form an outer surface of the liquid crystal display unit by using the washing solution.